The present invention relates generally to lasers based on periodic structures, and more particularly to periodic chiral lasers utilizing a single chiral element.
Semiconductor coherent laser beam sources have found many industrial and commercial applications in recent years. For example, lasers are used in telecommunications, in optically readable media pickups that are used in CD players, CD ROM drives and DVD players, and in medical imaging. In particular, wide area coherent lasers would be very useful in holographic displays, in communication systems and in information processing. However, previously known semiconductor lasers have a number of disadvantages. For example, traditional semiconductor lasers, such as ones used in CD players, emit light from the edge of a chip, so it is necessary to cleave a wafer into chips and package the chip before knowing if the laser functions properly. Other types of light sources, such as LEDs do not provide the performance needed for certain applications.
Vertical Cavity Surface Emitted Lasers (hereinafter xe2x80x9cVCSELsxe2x80x9d) have been developed to address the need for a more advanced, higher quality laser that can function well in a variety of applications. VCSELs have superior performance to edge-emitting lasers but at a significantly greater cost. VCSELs emit light vertically from the wafer surface, like LEDs. As a result, their fabrication and testing is fully compatible with standard I.C. procedures and equipment, so that arrays of VCSELs are feasible. Additionally, VCSELs are much faster, more efficient, and produce a smaller divergence beam than LEDs.
The VCSEL structure leads to a host of performance advantages over conventional semiconductor lasers.
1) small size
2) low power consumption
3) Two-dimensional array capabilities
In contrast to conventional edge-emitting semiconductor lasers, surface-emitting VCSELs have a radially symmetric Gaussian near-field, greatly simplifying coupling to optical elements or fibers. In addition, VCSEL technology allows the fabrication of two-dimensional laser arrays.
However, VCSELs suffer from a number of disadvantages. The manufacture of VCSELs requires sophisticated and expensive microfabrication. Since single-pass gain in thin layer semiconductor lasers is low, VCSELs incorporate highly reflective dielectric stacks which are integrated into the laser as Bragg reflectors. These consist of alternating layers of dielectric material, which are grown using methods of molecular beam epitaxy (MBE). This ensures a close match of the atomic lattice structures of adjacent layers. Alternating atomically ordered layers of materials with different electronic characteristics are thereby produced. The interfaces between the layers must be digitally graded and doped to reduce the electrical resistance.
Much work has been done to improve the performance of VCSELs by increasing the number of layers and/or the dielectric constant contrast between alternating layers. However, this approach makes the fabrication more expensive and difficult. There is also a limit to the number of layers determined by the absorption in these layers. While VCSELs can be manufactured in two-dimensional arrays, there has been great difficulty in achieving uniform structure over large areas and in producing arrays of large area. The materials typically used for VCSELs do not have the desired low absorption and high index contrast over a broad frequency range. In particular, it is difficult to achieve high reflectivity in the communication band around 1.5 microns.
In addition, VCSELs cannot be tuned in frequency since their periods cannot be changed. The density of photon modes is not changed appreciably by use of multilayer Bragg reflector with low refractive index contrast and the gain cannot be improved in a VCSEL system. Also, an external device must be used to control the polarization of the light.
Periodic chiral structures may be advantageously substituted for VCSELs to maximize performance and reduce device cost and complexity. One advantageous chiral structure utilizes an electro-luminescent emitter layer sandwiched between two cholesteric liquid crystal layers, that are in turn sandwiched between two electrodes connected to a current source. Several advantageous embodiments of such a structure are disclosed in the commonly assigned U.S. patent application Ser. No. 09/468,148, entitled xe2x80x9cChiral Laser Apparatus and Methodxe2x80x9d. However, a laser with two CLC layers suffers from some drawbacks. First, in such a two-CLC structure, at least one of the CLC layers must be polymeric making it difficult to produce long range correlation in the CLC structures. Second, in certain applications, such as when the chiral laser is integrated together with Thin Film Transistors into a device, direct access to one side of the emitting layer is difficult or impossible to achieve.
It would thus be desirable to provide a periodic laser apparatus and method that produces a coherent laser beam utilizing only a single chiral layer.
This invention relates to use of a single chiral structure combined with an excitable light-emitting material and a quarter wave plate to produce coherent lasing with performance similar to that of an ideal two chiral layer laser. Lasing action from the inventive chiral laser is achieved by placing an electro-luminescent emitting layer with properties of a quarter wave plate adjacent to a layer of a chiral material, for example, a cholesteric liquid crystal (CLC), between two electrodes. The electrode connected to the quarter wave plate emitting layer is highly reflective and serves as a source of electrons (for example, an aluminum electrode), while the second electrode, connected to the hole-transporting CLC, is a source of holes. An external current source is connected to the electrodes, and when a current is passed between the electrodes, the reflecting electrode releases electrons into the emitting layer, while the second electrode releases holes that are transported by the chiral layer into the emitting layer. Because the laser beam will be transmitted through the second electrode, the second electrode is preferably transparent. The recombination of electrons and holes in the emitting layer produces luminescence that initiates lasing. Optionally, the emitting layer can be optically excitable rather than electro-luminescent in which case the reflective electrode is replaced by a mirror layer while the second electrode is eliminated. Excitation of the emitting layer is produced by optical pumping. In alternate embodiments of the invention, the emitting quarter wave plate layer is composed of a separate quarter wave plate and an emitting layer.
The essence of the inventive chiral laser is that light emissions from the emitting layer oscillate between the CLC and the reflective electrode (or mirror) and repeatedly pass through the quarter wave plate emitting layer with a closed photon trajectory due to the quarter wave plate properties of the emitting layer and are incrementally amplified by the emitting layer resulting in lasing emission through the CLC layer.
The inventive apparatus and method advantageously overcome the drawbacks of previously known chiral lasers in that only a single chiral layer is utilized, greatly simplifying construction and reducing the cost. Furthermore, the inventive laser allows access to one side of the emitting layerxe2x80x94a feature important in many applications.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.